STROPHODONTiE OP THE UPPER HELDERBERG GROUP. 
91 
nulated. Dorsal valve moderately concave on the upper and middle part 
of the shell, and abruptly deflected or geniculate towards the front. 
Surface marked by fine radiating striae, which are somewhat unequal on 
the upper and middle portions of the ventral valve, but are fine and 
regular towards the margin. In some specimens of the dorsal valve, 
the striae show a tendency to the alternation of three or four finer ones 
with a distinctly stronger one between; hut in the greater number of 
specimens, the striae are fine, close, and mostly equal in strength. The 
radiating striae, when perfect, are crenulated by extremely fine con¬ 
centric striae : sometimes a few obscure concentric wrinkles mark the 
umbonal region, and some specimens show undefined longitudinal folds. 
In the ventral valve, the occlusor muscular impressions are strongly 
marked in elongate semielliptical imprints; while the divaricator muscu¬ 
lar impressions are elongate flabelliform, often reaching half the length 
of the valve. The specimen figured on Plate xiv is shorter than usual. 
In the dorsal valve' the muscular impressions are strongly marked, and 
divided by a low median ridge, which is continued above as a rather 
strong bifurcate cardinal process : hinge-line rather coarsely crenulated. 
This shell differs from S. inequiradiata, in attaining a larger size, and in being 
more gibbous on the ventral valye. The strige are finer and more equal, and the 
muscular impressions larger ; while the vascular impressions, so common in casts 
of that shell, have not been observed in this one. 
Its fine and nearly equal strige, as well as its greater gibbosity, distinguish it 
from S. patersoni. It approaches very nearly the S. concava of the Hamilton group; 
and the two have so many characters in common, that I have hesitated to continue 
them as distinct species. A larger collection of the limestone specimens are 
needed, before a satisfactory comparison can be made. 
The figure 2 a on Plate xiv illustrates the exterior of a dorsal valve, where the cardinal 
extremities are auriculate. The profile view, figure 2 b, is a little less gibbous than 
usual. 
The interior of the dorsal valve is shown in figure 2 c ; and the cardinal view, figure 2 d, 
shows the muscular impressions of the ventral valve; 
Geological formation and locality. This species occurs in the Schoharie grit in 
Albany and Schoharie counties, and not rarely in the Cornifereus limestone in 
nearly all localities of this formation throughout the State; and I have a specimen 
